1. Field of the Invention
The present inventions relate to electrical circuits and, more particularly, circuits for operation of light emitting diodes.
2. Brief Description of the Related Art
A step down voltage regulator is commonly used in systems that use high input voltages such as 24V, 48V, 120V or higher that must be locally converted to a lower voltage such as 15V, 12V or 5V with very little power loss. A Buck regulator is an example of a DC to DC step-down voltage regulator. The Buck regulator, unlike linear dissipative regulators, can be used to step DC voltage down to a lower DC voltage of the same polarity. For stable DC input voltages such as in battery powered systems, traditional Buck regulators provide a very efficient form of power conversion from higher to lower voltages.
A Buck regulator in electrical communication with a discontinuous DC voltage source is illustrated in FIG. 1. The Buck regulator takes advantage of the energy storage characteristics of two passive components, a capacitor for voltage storage and an inductor for current storage. The Buck regulator alternates between two modes of operation, an ON mode and an OFF mode. In the ON mode, the capacitor, the inductor, and the LED are connected to the source voltage, which charges the inductor and the capacitor and powers the LED. In the OFF mode, the capacitor, the inductor, and the LED are disconnected from the voltage source, and the inductor and the capacitor are discharged into the LED.
However, Buck regulators may have some disadvantages. Voltage ripple is the phenomenon where the voltage rises during the ON mode and falls during the OFF mode. Several factors contribute to voltage ripple including, but not limited to, switching frequency, capacitance, load, and any current limiting features of the control circuitry. At the most basic level, the output voltage will rise and fall as a result of the charging and discharging of the capacitor. Qualitatively, as the capacitance increases, the magnitude of the voltage ripple decreases. The capacitance is generally limited by cost, physical size and non-idealities of various capacitor types. The magnitude of the voltage ripple also decreases as the switching frequency increases. However, the ability to increase the switching frequency is limited. Switching losses reduce efficiency, and non-ideal switching characteristics of the free-wheeling diode can also reduce efficiency and may raise EMI concerns.
When the voltage source is a rectified AC voltage with a sinusoidal waveform, the switching frequency may be based on the frequency of the voltage source. A large capacitance may be required to reduce ripple to an allowable level. Buck regulators typically use an electrolytic capacitor to reduce voltage ripple to the allowable level. However, the use of electrolytic capacitors introduces several parasitic elements which can make buck regulator performance less than ideal. Large electrolytic capacitors have a large equivalent series resistance (ESR) which directly affects the performance and efficiency of any switching regulator. Electrolytic capacitors have high ESR because the dielectric contains a liquid-based electrolyte. At low operating temperatures, the ESR of a typical aluminum electrolytic may increase by 40 times as the temperature drops from 25° C. to −40° C., which will typically cause the capacitor to quit working. At high temperatures and/or high operating current, the liquid present in the capacitor may evaporate and the ESR increases due to internal heat generation. Unfortunately, as the ESR goes up, so does the internal heat generation, which can cause the capacitor to fail. In extreme cases, the electrolyte can actually boil and cause the capacitor to explode.
A light emitting diode (LED) may operate in two different modes, either a constant current mode, which is the most common, or a pulsed current mode. In the constant current mode, a constant DC current is delivered to the LED. In the pulsed current mode, pulses of regulated current are delivered to the LED. The pulsed current mode with specified maximum duty cycle and maximum pulse width is recommended by majority of the LED manufacturers, because the LED may cool between pulses, which may prolong the life of the LED.
Accordingly, a need exists for a device that may deliver a regulated pulsed current to an LED, and that may avoid the problems associated with devices such as Buck regulators.